numerical investigation on welding residual stress in a pwr- tae-kwang song-2010.pdf

8/11/2019 Numerical Investigation on Welding Residual Stress in a PWR- Tae-Kwang Song-2010.pdf

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doi: 10.1111/j.1460-2695.2010.01480.x

Numerical investigation on welding residual stresses in a PWRpressurizer safety/relief nozzle

T A E - K W A N G S O N G 1, H O N G - R Y U L B A E 1, Y U N - J A E K I M 1 a n d K Y U N G - S O O L E E 2

1Department of Mechanical Engineering, Korea University, 1-5 Ka, Anam-Dong, Sungbuk-Ku, Seoul 136-701, Korea, 2Korea Electric Power Research

Institute, Yusung-gu, Daejon 305-380, Korea

Received in final form 11 January 2010

A B S T R A C T This paper presents finite element simulation results of residual stresses in dissimilarmetal welds of a PWR pressurizer safety/relief nozzle. The present results are believedto be significant in two aspects. The first one is to consider the effect of the presenceof similar metal welds on resulting residual stresses. The second one is the mitigationeffect of the overlay welding thickness on residual stresses. After dissimilar metal welding,tensile residual stresses are present both at the inner surface and at the outer surfaceof dissimilar metal welds. Adjacent similar metal welding, however, decreases residual

stresses to compressive ones at the inner surface of dissimilar metal welds, possibly dueto the bending mechanism caused radial contraction of the weld. At the outer surface ofdissimilar metal welds, similar metal welding increases residual stresses. Overlay weldingfurther decreases residual stresses at the inner surface of dissimilar and similar metal

welds, but increases slightly residual stresses at the outer surface.

Keywords finite element simulation; PWR pressurizer safety/relief nozzle; weld over-lay; welding residual stress.

I N T R O D U C T I O N

Recently stress corrosion cracks were found in dissimi-lar metal welds of some pressurized water reactor (PWR)nuclear plants.15 As a result, several mitigation methodshave been discussed,57 one of which is the weld over-lay.5,810 As the weld overlay method has been applied topiping systems in boiling water reactor components oversome decades,11,12 its effectiveness in mitigation is wellappreciated. In addition, the weld overlay can also pro-

vide other benefits, for instance, improved inspectabilityand structural reinforcement effects due to an increasingthickness.5,10,12 Although some guidelines on the weldoverlay for dissimilar metal welds in pressurized waterreactor nuclear plants are available,1315 numerical simu-lations are desirable to assess the weld overlay effect onresidual stresses, as the weld overlay process is expensive.Furthermore such numerical simulations can be used tohelp optimise the weld overlay process.Welding residual stress simulation of stainless pipes us-

ing the finite element (FE) analysis is relatively well estab-lished, and accordingly many papers have been published

Correspondence : Yun-Jae Kim. E-mail: [email protected]

in the literature.1620 Some relevant works are briefly de-scribed here. Brickstad and Josefson16 performed a para-metric study for multipass circumferential butt-weldingof stainless steel pipes to quantify residual stresses andtheir sensitivity to variation in weld parameters. Yaghiet al.17 presented another parametric study on effects ofthe thickness and radius-to-thickness ratio on residualstresses in butt-welded stainless pipes. Related to repair

welds and overlay welding for dissimilar metal welds inPWR plants, many works have been published in Ameri-can Society of Mechanical Engineers, Pressure Vessel andPipingDivision (ASME PVP) conferences.46,8,9 One im-portant point is that all these works assume an isolated

weld. Typical PWR components, however, have similarmetal welds adjacent to dissimilar ones. An example isshown in Fig. 1 where a pressurizer safety/relief nozzle inone Korean PWR plant is schematically shown. It showsthat a nozzle is connected to a stainless pipe by dissimilarand similar metal welds. Intuitively the presence of sim-ilar metal welds could alter residual stresses in dissimilarmetal welds. This is because, during similar metal weld-ing, thermal contraction of deposited weld metal in thehoop direction effectively applies a tourniquet ring load

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Fatigue & Fracture of

Engineering Materials & Structures


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690 T A E - K W A N G S O N G et al.

Fig. 1 Schematic illustrations of a

pressurizer safety/relief nozzle; (a) overall

geometry and materials, and (b) the

geometry considered in this work and

relevant dimensions (in mm).

on the safe-end, which in turn can provide through-wallbending stresses to dissimilar metal welds. Moreover, asthermal contraction of deposited weld metal essentiallygives displacement controlled load, the distance betweendissimilar and similar metal welds (or the safe-end length)could affect residual stresses in dissimilar metal welds.

Therefore, it would be important to quantify the effect ofthe presence of similar metal welds on residual stresses ina dissimilar metal weld before discussing the weld overlayeffect on residual stresses.

The objectives of this paper are twofold. The first oneis to estimate welding residual stresses in dissimilar metal

welds for a typical PWR plant, via FE simulations. A par-ticular emphasis is put on the effect of similar metal weldsadjacent to dissimilar ones on residual stresses. The sec-ond objective is to quantify the weld overlay effect onresidual stresses. To be realistic, the actual geometry forthe pressurizer safety/relief nozzle in a Korean PWR nu-clear component is considered. Section 2 describes thegeometry and materials considered in this work, and

FE simulations are described in Section 3. Section 4presents results and discussion. This work is concluded inSection 5.

G E O M E T R Y A N D M A T E R I A L S

Geometry

Figure 1 depicts a pressurizer safety/relief nozzle in a typ-ical PWR nuclear plant, considered in this work. Relevant

materials and dimensions are also included in the figure.A nozzle made of SA 508 Cl. 2a ferritic steel is welded tothe safe-end made of F316L stainless steel, by means of

Alloy 82 buttering laid on the SA 508 nozzle and Alloy82/182 groove welding. The F316L pipe is then weldedto another pipe made of TP304L stainless steel, by meansof similar metal welding using Alloy ER 308L. It shouldbe noted that the geometry considered in this work con-sists not only of dissimilar metal welds but also of sim-ilar metal welds close to dissimilar ones. The welding

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N U M ER I C AL IN V E ST I G AT I O N O N W E L DI N G R E S ID U A L S T R E SS E S 693

Fig. 2 The FE mesh for dissimilar metal

welding simulation, showing beads and

welding sequence.

Fig. 3 The FE mesh for similar metal

welding simulation, showing beads and

welding sequence.

residual stresses and thus is conservative for the present

purpose.26

FE simulations

Dissimilar metal welds are simulated as follows. For tran-sient thermal analysis, heating time and input are esti-mated based on the WPS shown in Table 1. For thebuttering and cladding, their thicknesses (20 mm for thebuttering layer and 4.5 mm for the cladding) and rele-

vant material properties are considered. However, possi-ble residual stresses due to buttering and cladding are notconsidered, as they would be minimal after post weld heat

treatment.4,22 Sixteen beads are simulated to be consis-tent to the WPS. Figure 2 shows the FE mesh with beadnumbers showing welding sequence. Regarding kinematicboundary conditions, the roller boundary and clampedconditions are applied to the ends of the nozzle and safe-end, respectively (see Fig. 2), to reflect actual (shop-weld)conditions. That is, the axial displacement of the nozzleend is restrained, and the end of the safe-end is fixed.

After simulation, the restraint imposed to the end of thesafe-end was released.

For similar metal welds, the overall simulation proce-

dures are similar to those for dissimilar metal welds. Ac-cording to the WPS, a total of eleven beads are simu-lated, as shown in Fig. 3. Kinematic boundary conditionsare worth noting. During welding, the roller boundarycondition is applied to the end of the nozzle, as for thedissimilar welding simulation (Fig. 3). The free bound-ary condition is imposed to the end of the pipe, and thusno reaction force from the adjacent pipes is considered(Fig. 3).The hydrotest is simulated after similar metal welding

simulation. The test pressure should be 1.25 times designpressure according to ASME Section III.27,28 For the par-ticular nozzle considered in this work, the design pressure

was 17.2 MPa, and thus hydrostatic pressure of 21.5 MPawas applied to the nozzle. Hydrostatic pressure was ap-plied as a distributed load to the inner surface of the FEmodel, together with an axial tension equivalent to theinternal pressure applied to both ends to simulate closingends.To simulate overlay welding, its thickness and length

should be assumed first. Figure 4a shows a schematic il-lustration of weld overlay. For boiling water reactors, acriterion for the thickness and length of weld overlay is

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Fig. 4 (a) Schematic illustration of overlay

welding, and (b) FE mesh for overlay

welding simulation.

well established.11,12,15 For pressurized water reactors, thecode case N-74013 provides guidance, suggesting that theoverlay thickness,tWOL, should be greater than one-thirdof the pipe thickness. For piping components, the thick-ness can be easily defined. However, for the nozzle, thick-nesses vary with the spatial location. In this work, the pipe

thickness is defined as the average thickness of the dissim-ilar metal welds, denoted as tDMW(Fig. 4a). Accordinglythe overlay thickness is taken to be one-third of tDMW,tWOL = tDMW/3. In the code case N-740, the weld over-lay is recommended to cover 0.75(ro tDMW)

0.5 (whereroandtDMWdenote the outer radius and pipe thickness, re-spectively) from the weld end at the outer surface. Whenonly dissimilar metal welds are present, this implies thatthe length of weld overlay should be the sum of 1.5(rotDMW)0.5 and the weld width.13,15 In the present problem,however, similar metal welds are very close to dissimilarones, and thus the weld overlay is assumed to extend by0.75(rotDMW)

0.5 from similar metal welds, as depicted in

Fig. 4a. Regarding materials, overlay welding is typicallyperformed using Alloy 690 of which relevant mechanicalproperties for welding simulations are not available to theauthors. As mechanical/physical properties for Alloy 690are known to be similar to Alloy 600, relevant proper-ties for Alloy 600 are used for simulations. The overlay

welding is assumed to use GTAW and to have three lay-ers. The welding direction is taken from the nozzle tothe pipe, as indicated in Fig. 4a. The FE mesh is shownin Fig. 4b. Finally it is recommended to perform weld

overlay with the water (instead of air) condition insidethe nozzle-pipe system to improve the mitigation effectof the weld overlay due to the larger temperature gradientthrough the pipe thickness.12 Such a condition is consid-ered in this work, and the heat conductivity for water istaken to be 5000 (W/m C) which is five hundred times

that of air.15

It should be noted that the air condition wasalso performed and the resulting residual stresses werefound to be similar. This is probably due to the fact thatthe radius-to-thickness ratios of the pipes for the presentproblem are small.

Comparison with existing results

To gain confidence of present FE simulations, sensitivityanalyses were performed for parameters related to resid-ual stress simulations. The findings from sensitivity anal-

yses were overall consistent to those reported in otherworks.16,20,26 Further confidence was gained by compar-

ing present results with published results, as briefly de-scribed below.

It was argued in Refs [16,17] that the sizes of the fu-sion line and heat affected zone could be indicators forthe quality of transient thermal analysis. In other words,parameters related to thermal analysis can be adjusted toproduce reasonable sizes of the fusion and heat affectedzones. Note that the heat-affected zone here is definedby the region experiencing the maximum temperature of800900 C during welding. In this paper, parameters



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Fig. 5 Variations of peak temperature along the HAZ line and temperature contours: (a) dissimilar metal welding simulation and (b) similar

metal welding simulation. In contours, brighter regions indicate the region with temperatures above 900 C.

related to thermal analysis was chosen to produce the2.5 mm heat affected zone size. Figure 5 show plots of

the peak temperature at the HAZ line during welding indissimilar and similar metal welding.

In the literature, welding residual stresses for dissimi-lar metal welds, resulting from the FE analysis, were re-ported in Ref. [1]. The materials considered in Ref. [1]are the same as the present ones, and furthermore thegeometry is very close, as listed in Table 3. Figure 6 com-pares residual stresses resulting from the present analysis

with those from Ref. [1], showing overall good agree-ments. Note that simulations were performed according

Table 3 Comparison of dimensions

Present analysis EPRI (5

)DMW DMW

t(mm) 33 40

ri(mm) 65.9 62.6

ri/t 2.00 1.57

to the conditions in Ref. [1]. Materials were assumed tobe elastic-perfectly plastic. Similar metal welds were notconsidered, and kinematic boundary conditions were alsothe same. Minor differences shown in Fig. 6 are believed



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Fig. 6 Through-thickness variations of (a) axial and (b) hoop residual stresses along the centre line in dissimilar metal welds; comparison ofpresent FE results with those in Ref. [1].

Fig. 7 Three paths along which axial and

hoop residual stresses are extracted. Due to

the un-symmetrical geometry of dissimilar

metal welds, the path A is not vertical.

to result from the material data used in simulations, whichare not given in Ref. [1].

R E S U L T S A N D D I S C U S S I O N

In this section, residual stress distributions resulting frompresent FE simulations are presented. Before presenta-tion, it is important to note locations where stresses areextracted and components to be reported. Regarding lo-cations, stress distributions are reported along three dif-ferent paths, as depicted in Fig. 7. The paths A and Care centre lines within dissimilar and similar metal welds,respectively, whereas the path B follows the inner sur-

face. Regarding components, both axial and circumferen-tial (hoop) stress components are reported.

Residual stresses after dissimilar welding simulationFigure 8 shows FE (hoop and axial) residual stresses af-ter dissimilar metal welding. Along the thickness direc-tion from the inner toward the outer surface (path A),both axial and hoop residual stresses change from tensile,compressive and then to tensile. Although both axial andhoop stresses are tensile at both the inner and the outersurfaces, magnitudes at the outer surface are much largerthan those at the inner one. At the inner surface, residualstresses are less than 100 MPa, whereas those at the outer



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Fig. 8 Variations of axial and hoop residual stresses after dissimilar metal welding: (a) along the path A (through-thickness), and (b) alongthe path B (longitudinal).

surface can be as much as 400 MPa. Longitudinal vari-ations of residual stresses at the inner surface (along thepath B) are shown in Fig. 8b. In the buttering layer, theaxial residual stress is compressive to the nozzle side, butis tensile to the dissimilar weld side. In the safe-end, theaxial stress is overall compressive. Hoop residual stressesare compressive in the buttering layer but are tensile inthe safe-end.

Residual stresses after similar welding simulation

Figure 9 compares residual stresses before and after sim-ilar metal welding. Figure 9a shows that similar metal

welding does alter both shapes and magnitudes of resid-ual stresses in dissimilar metal welds. Similar metal weld-ing decreases residual stresses at the inner surface, butincreases those at the outer surface. At the inner sur-face, both the axial and hoop stresses are decreased by200 MPa, and the resulting residual stresses are com-pressive. At the outer surface, on the other hand, theaxial residual stress is increased by200 MPa, but the

hoop residual stress is not much affected. Such effectsare possibly due to the bending mechanism caused bythe proximity of the similar metal weld. This result isconsistent to experimental and numerical findings in theliterature.29,30

The effect of similar metal welding on residual stressesat the inner surface (along the path B) is shown inFig. 9b. The similar metal welding process decreasesresidual stresses both in the buttering layer and in thesafe-end, and resulting (axial and hoop) residual stresses

are compressive. Through-thickness variations of resid-ual stresses in similar metal welds (along the path C) areshown in Fig. 10. It shows that overall shapes are similarto those in dissimilar metal welds, shown in Fig. 8a. Atthe inner surface, small compressive residual stresses arepresent, whereas at the outer surface, residual stresses aretensile.

Effect of hydrotest on residual stresses

Figure 11 shows residual stress distributions along thepath A and path C after the hydrotest. It shows that theeffect of overload in the hydrotest on residual stresses isminimal. The results for the path B are not shown, asthey also show almost identical results. Such a minimaleffect is due to the fact that the radius-to-thickness ratiofor the present problem is quite small, corresponding tothick-walled pipes.6

Effect of overlay welding on residual stresses

Figure 12 shows the effect of overlay welding on through-thickness variations of residual stresses in dissimilar metal

welds (along the path A). As expected, overlay weldingdecreases residual stresses at the inner surface. The axialand hoop residual stresses are decreased by100 and200 MPa, respectively, showing the mitigation effect ofoverlay welding on residual stresses at the inner surface. Atthe outer surface, however, residual stresses are increasedup to 500 MPa after overlay welding. One interestingpoint is that residual stresses at the inner surface tend to



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Fig. 9 Variations of axial and hoop residual stresses after similar metal welding: (a) and (b) along the path A (through-thickness); (c) and

(d) along the path B (longitudinal).

saturate after the first pass of weld overlay. The secondand third passes do not change residual stresses at theinner surface.

Figure 13 shows the effect of weld overlay on longitu-dinal variations of residual stresses at the inner surface(along the path B). As shown, residual stresses at the innersurface are overall decreased with weld overlay. The effectof weld overlay on residual stresses is more pronouncedin the safe-end and in similar metal welds. For instance,in similar metal welds, (axial and hoop) residual stressesdecrease by up to 300 MPa.The effects of weld overlay on through-thickness varia-

tions of residual stresses in similar metal welds (along thepath C) are shown in Fig. 14. It shows the overall effects

are similar to those in dissimilar metal welds, as shown inFig. 12. Overlay welding decreases residual stresses at theinner surface, but increases those at the outer surface.

C O N C L U D I N G R E M A R K S

This paper presents variations of residual stresses in dis-similar metal welds of a PWR pressurizer safety/reliefnozzle, via elastic-plastic FE simulations. As the nozzleconsidered in this work consists of dissimilar metal welds,the safe-end and similar metal welds, actual fabricationprocedures (including dissimilar metal welding, similarmetal welding and the hydrotest) are simulated to quan-tify residual stresses. Furthermore the mitigation effectof overlay welding is also investigated. Present results are



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Fig. 10 Variations of axial and hoop residual stresses along the

path C (through-thickness in similar metal welds) after similar

metal welding.

believed to be significant in two aspects. The first oneis to consider the effect of the presence of similar metal

welds on resulting residual stresses. The second one is tothe mitigation effect of the overlay welding thickness onresidual stresses.

Important findings from this work can be summarisedas follows. After dissimilar metal welding, tensile resid-ual stresses are present both at the inner surface and atthe outer surface of dissimilar metal welds. Stresses at theouter surface are much higher than those at the innerone. However, similar metal welding decreases residual

stresses at the inner surface of dissimilar metal welds,possibly due to the bending mechanism caused by weld-

ing. As results, residual stresses at the inner surface be-come compressive. At the outer surface of dissimilar metal

welds, similar metal welding increases residual stresses.As explained in Introduction, such through-wall bendingstresses are due to tourniquet ring load resulting fromthermal contraction of deposited weld metal during simi-

lar metal welding. Residual stresses are not affected by thehydrotest, possibly due to the fact that the mean radius-to-thickness ratios are very small. The weld overlay effecton residual stresses is similar to those of similar metal

welding. Overlay welding decrease residual stresses atthe inner surface of dissimilar and similar metal welds.Residual stresses at the outer surface, on the other hand,are increased slightly. One interesting point is that thethickness of overlay welding is currently recommended byone-third of the pipe thickness. The present results sug-gest that one-third of the recommended overlay thickness

would be sufficient to mitigate residual stresses at the in-ner surface of dissimilar metal welds. However, it should

be noted that this work does not consider any crack in dis-similar metal welds, and the presence of the crack mightchange conclusions.

Finally, limitations of present results should be noted.To be realistic, this work considers the specific geome-try and materials (the pressurizer safety/relief nozzle ina Korean PWR nuclear plant). One notable point in ge-ometry is that overall radius-to-thickness ratios are closeto 2, corresponding thick-walled pipes. Another pointis that the safe-end length (the distance between dis-similar and similar metal welds) is relatively short. It is

well known that welding residual stresses depend on the

radius-to-thickness ratio. Moreover the effect of simi-lar metal welding on residual stresses in dissimilar metal

Fig. 11 Variations of axial and hoop residual stresses after hydrotest: (a) along the path A and (b) along the path C.



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Fig. 12 Effect of overlay welding on variations of (a) axial and (b) hoop residual stresses along the path A (the centre line within dissimilarmetal welds).

Fig. 13 Effect of overlay welding on variations of (a) axial and (b) hoop residual stresses along the path B.

welds should depend on the distance between dissimilarand similar metal welds, as such an effect results frombending due to welding. Depending on the design, theradius-to-thickness ratio and the distance between dis-similar and similar metal welds can vary, and thus the re-sulting residual stresses can be different. Therefore moreparametric study is needed to quantify residual stresses fordissimilar metal welds in PWR nuclear plants. A prelimi-nary study has suggested that residual stresses in dissimilarmetal welds depend not only on the radius-to-thickness

ratio but also on the distance between dissimilar and sim-ilar metal welds. More detailed results will be reportedseparately. The last point is that this work does not con-sider possible effects of repair welding and the presenceof a crack in dissimilar metal welds. It is known that repair

welding increases residual stresses at the inner surface ofdissimilar metal welds, which promotes stress corrosioncracking. The presence of a crack might affect the mitiga-tion effect of overlay welding. Works on these issues arein progress and results will be reported later.



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Fig. 14 Effect of overlay welding on variations of (a) axial and (b) hoop residual stresses along the path C (the centre line within similar

metal welds).

Acknowledgements

This research is supported by KESRI(R-2007-2-066) andby Korea Science and Engineering Foundation, Engi-neering Research Center (No. 2009-0063170) and GrantNo. M207AE030001-08A0503-00111.

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numerical investigation on welding residual stress in a pwr- tae-kwang song-2010.pdf

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